1. Technical Field
The present invention relates to a channel estimation technique that is used to perform demodulation processing on a transmission signal generated by multiplexing a plurality of modulated carriers.
2. Background Art
With the ability to efficiently use frequency, an Orthogonal Frequency-Division Multiplexing (OFDM) method is a transmission system suitable for high-speed data communication. The OFDM method is also a digital multi-carrier transmission system utilized as a Japanese digital terrestrial broadcasting system (Integrated Services Digital Broadcasting-Terrestrial transmission, ISDB-T) and a European digital terrestrial broadcasting system (Digital Video Broadcasting-Terrestrial transmission, DVB-T) (for example, see Non-patent Documents 1 and 2).
In the OFDM method, after complex signals are allocated to a plurality of mutually orthogonal sub-carriers, a transmission signal is generated by performing the inverse fast Fourier transform on the modulated carriers in each predetermined symbol duration, then the generated transmission signal is transmitted. In the ISDB-T and DVB-T systems, a signal called a guard interval is inserted into the transmission signal for the purpose of guarding against the InterSymbol Interference (ISI) caused by multipath, Signal Frequency Network (SFN), and the like.
In the ISDB-T system, signals such as scattered pilot signals, control signals, additional signals and continual pilot signals are allocated to certain carriers and transmitted along with data. The amplitudes and phases of the scattered pilot signals, which are used as references for equalization, are known to a receiver. The control signals are called Transmission and Multiplexing Configuration Control (TMCC) and transmit information on transmission parameters and the like. The additional signals are called Auxiliary Channel (AC) and transmit additional information. The amplitudes and phases of the continual pilot signals are known to the receiver.
In the DVB-T system, signals such as scattered pilot signals, control signals and continual pilot signals are allocated to certain carriers and transmitted along with data. The amplitudes and phases of the scattered pilot signals, which are used as references for equalization, are known to the receiver. The control signals are called Transmission Parameters Signalling (TPS) and transmit information on transmission parameters and the like. The amplitudes and phases of the continual pilot signals are known to the receiver.
The receiver extracts, from the received OFDM signal, a portion having a useful symbol length, and performs the Fourier transform on the extracted portion. This process transforms the stated portion having the useful symbol length, which is a time domain OFDM signal, into a frequency domain OFDM signal. However, as the received OFDM signal has been affected by channel distortion, it is necessary to remove such channel distortion from the frequency domain OFDM signal. The processing of removing the channel distortion is called equalization or channel equalization. In equalization, channel characteristics are estimated by using scattered pilot signals included in the frequency domain OFDM signal, then the channel distortion is removed by using the estimated channel characteristics.
With reference to FIGS. 17(a), 17(b) and 17(c), a description is now given of channel estimation processing that is performed with use of scattered pilot signals (hereinafter, “SP signals”) in an exemplary case where the ISDB-T system is used.
As shown in FIG. 17(a), the SP signals are allocated as follows in the ISDB-T system. In each symbol, an SP signal is allocated to every 12th carrier in the carrier (frequency) direction. In each carrier, an SP signal is allocated to every 4th symbol in the symbol (time) direction. A channel estimation unit included in the receiver (i) extracts SP signals from the frequency domain OFDM signal, (ii) generates SP signals that are identical to SP signals inserted by a transmitter, (iii) divides each of the extracted SP signals by a corresponding one of the generated SP signals, and (iv) considers each result of this division as an estimated value of channel characteristics (hereafter, “an estimated channel characteristics value”) at a corresponding one of SP signal positions. Then, the channel estimation unit interpolates, in the symbol (time) direction, the estimated channel characteristics values at the SP signal positions as shown in FIG. 17(b). Thereafter, the channel estimation unit interpolates, in the carrier (frequency) direction, each of the estimated channel characteristics values at a corresponding one of symbol positions in each carrier to which an SP signal is allocated using a certain symbol as shown in FIG. 17(c). This way, an estimated channel characteristics value is obtained for every cell in each carrier and each symbol.
Generally, interpolation in the frequency direction is performed by using an interpolation filter. Before performing the interpolation by using the interpolation filter, estimated channel characteristics values are obtained only for every 3rd carrier. As a result, the estimated channel characteristics values include an aliasing component at intervals of Tu/3 as shown in FIG. 18, where Tu denotes a useful symbol duration. Here, the best transfer function of the interpolation filter allows more signal components, including signal components originating from multipath and SFN, to pass through the interpolation filter, thus removing more noise components. However, as channel status varies between different channels, the best transfer function cannot be uniquely determined. In view of this, if the transfer function of the interpolation filter is controlled in accordance with channel status, then the accuracy of channel estimation will be improved. Note, Non-patent Document 3 provides a discussion on the best filter as well as interpolation in the time direction.
Methods of controlling the transfer function of an interpolation filter in the frequency direction have been disclosed in the prior art. For example, Patent Document 1 discloses a method of controlling the transfer function of an interpolation filter by using an estimated value of a channel response, and Patent Document 2 discloses a method of controlling the transfer function of an interpolation filter by using signal qualities.